1. Field of the Invention
The present invention relates to resist materials for fabricating a fine pattern and fabrication method of a resist material. This application is based on patent applications Nos. Hei 8-166607, Hei 8-242560, and Hei 9-038538 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Pattern fabrication in relation to semiconductor integrated devices represented by ULSI (Ultra Large Scale Integrated Circuit) is performed such that a thin film layer of a resist material (often abbreviated as a "resist", hereinafter), which is sensitive to high energy beams such as Ultra-violet light (often abbreviated as a "UV", hereinafter), X-ray, or electron beams, is deposited on a semiconductor substrate, and then the resist is irradiated with such high energy beams and is developed.
FIGS. 3A-3B are diagrams for explaining processes for fabricating a pattern using a conventional resist material. In FIGS. 3A-3B, reference numeral 1 indicates a (semiconductor) substrate, reference numeral 2 indicates a resist film, reference numeral 3 indicates high energy beams (such as UV, X-ray, or electron beams), and reference numeral 4 indicates reactive etching species. Accordingly, it is necessary for the resist materials (i) to have high sensitivity to high-energy beams in consideration of quick pattern fabrication, (ii) to be keenly sensitive to the high energy beams so as to obtain high pattern-resolution capability, and (iii) to have high etching resistance in the etching of semiconductor-substrate. Generally, the thinner the resist film is, the smaller the spreading of the high energy beam is in the resist film; thus, pattern resolution is increased. Similarly, due to thinner resist films, etched pattern transfer difference from a resist pattern becomes smaller; thus, fabrication precision in the substrate etching is improved. Therefore, the pattern fabrication has been performed with a resist film as thin as possible. In particular, in the research and development of most-advanced devices such as the next ULSI, or quantum-effect devices, pattern widths will be in a range between 10 nm.about.150 nm, thus thinning of resist films is much more necessary for realizing miniaturization or nano-fabrication and higher precision.
The resist used for such ultra-fine processing can be generally classified into the following 5 types:
(1) a resist comprising an alkaline soluble resin and a diazonaphthoquinone-compound as a photo-sensitizer; PA1 (2) an acrylic type polymer resist which degrades via main chain scission; PA1 (3) a resist material comprising an alkaline soluble resin and an azide as a photo-sensitizer; PA1 (4) a crosslinking type resist containing a chloromethyl group or an epoxy group; and PA1 (5) a chemical amplification resist comprising an alkaline soluble resin, an acid generator, and a dissolution controlling agent having an acid sensitive group. PA1 (1) a resist comprising an alkaline soluble resin and a diazonaphthoquinone-compound as a photo-sensitizer; PA1 (2) an acrylic type polymer resist which degrades via main chain scission; PA1 (3) a resist material comprising an alkaline soluble resin and an azide as a photo-sensitizer; PA1 (4) a crosslinking type resist containing a chloromethyl group or an epoxy group; and PA1 (5) a chemical amplification resist comprising an alkaline soluble resin, an acid generator, and a dissolution controlling agent having an acid sensitive group. PA1 (1) a first step of coating a substrate with a resist film; and PA1 (2) a second step of depositing particles whose major component is a cluster of carbon atoms on the resist film. PA1 (i) closely packing the resist film with carbon particles because free spaces among resist molecules formed during spin-coating process are filled up by the particles, and PA1 (ii) inhibiting penetration of etching reactive species into the resist film and enhancing the etching resistance of the resist film because the major component of the particles is a cluster of carbon atoms which has high etching resistance.
The resist material of type (1), generally used in LSI processing, is exposed to UV and the diazonaphthoquinone-compound as a sensitizer is subjected to a chemical change during UV exposure, by which solubility of the alkaline soluble resin is enhanced and a pattern is fabricated. As the alkaline soluble resin, a novolac resin, a phenol resin, poly(hydroxy styrene), and the like are used; however, the novolac resin is most commonly used. This type of resist has been used for pattern fabrication with a relatively thick film, approximately up to 200 nm.
The resist material of type (2) has mainly been used in the ultra-fine pattern fabrication of 200 nm or less. In this type, the acrylic main chains are cut by irradiating an electron beam, an X-ray, or UV having a wavelength of 300 nm or less, by which solubility of the resist is enhanced and a pattern is fabricated. (i) Poly(methyl methacrylate) (i.e., PMMA), (ii) ZEP (manufactured by Nippon Zeon Co.) which is a copolymer of .alpha.-chloro methacrylate and .alpha.-methyl styrene, and (iii) poly 2,2,2-trifluoroethyl .alpha.-chloro acrylate (e.g., EBR-9 manufactured by Toray Co.) are representative resist materials of this type. In this type of resist, the difference in solubility rates between exposed and unexposed areas is very large, hence very high resolution can be realized. Therefore, this type of resist is generally used in thin-film form so as to fabricate an ultra-fine pattern of 200 nm or less.
As an example of fabricating a fine pattern of 10-50 nm range by thinning the resist film, it has been reported, in "Fabrication of 5-7 nm wide etched lines in silicon using 100 keV electron-beam lithography and polymethylmethacrylate resist", Applied Physics Letters, Vol. 62 (13), pp. 1499-1501, Mar. 29, 1993, that a Si substrate is dry-etched through a mixture gas of SiCl.sub.4 and CF.sub.4 by using a 65 nm thick PMMA resist, a representative high-resolution resist. Another article, "Si nanostructures fabricated by electron beam lithography combined with image reversal process using electron cyclotron resonance plasma oxidation", Journal of Vacuum Science and Technology, Vol. B13 (6), pp. 2170-2174, November/December, 1995, also reported the use of a 50 nm thick ZEP resist, which is known to have resolution as good as PMMA and also to have relatively high etching resistance for oxygen plasma processing of substrate.
On the other hand, in the case of defining a relatively large nanometer pattern of 50-150 nm, necessary resolution can be achieved in the present circumstances by using a relatively thick PMMA or ZEP resist and high energy beams mentioned above. In this range of pattern size, it is rather important to process substrates without forming any defects during dry-etching; then, the resist thickness is increased to ensure necessary dry-etching resistance. Typical resist thickness for this purpose is in a range of 0.1-0.5 .mu.m.
The resist material of type (3) is normally used for UV or an electron beam exposure. This type of resist comprises the azide as a photo-sensitizer and the alkaline soluble resin, and the chemical change of azide, and the UV or electron beam exposure makes alkaline soluble resin insoluble, resulting in the formation of negative type patterns.
The resist material of type (4) comprises polymer resins containing the chloromethyl group or the epoxy group which has high crosslinking reactivity. In this type, the polymers are crosslinked with each other when they are irradiated by UV, an electron beam, or an X-ray, and the polymers become insoluble and a pattern is fabricated. This type of the resist is mainly used in negative-type pattern fabrication in which exposed areas remain.
The resist material of type (5) comprises the alkaline soluble resin, the acid generator, and the dissolution controlling agent (it may also be called a dissolution inhibitor) having the acid sensitive group. In this type, an acid is generated from the acid generator through irradiation of UV, electron beam, or X-ray, then, the acid reacts with the acid sensitive group of the dissolution controlling agent, by which the solubility of the alkaline soluble resin is changed and a pattern is fabricated. Since the reaction between the acid and the dissolution controlling agent proceeds via chain reaction scheme, very high sensitivity can be achieved in this type of resist. As the alkaline soluble resin, a novolac resin, a phenol resin, poly(hydroxy styrene), and the like are used. Additionally, in the chemical amplification type, there are some variations such that (i) the alkaline soluble resin functions as a dissolution controlling agent having an acid sensitive group, or (ii) a resin having an acid sensitive group reacts with an acid, making the resin alkaline soluble. However, in this type of resist, the acid becomes inactive due to water, ammonia, and the like included in the air, hence the resist sensitivity is considerably varied with the time between exposure and development. In order to stabilized the sensitivity, another polymer film to prevent the deactivators from penetrating in the resist film are overcoated on the resist.
Hereinbelow, common problems relating to the resists of the above (1)-(5) types will be explained.
In the conventional fine-pattern fabrication methods, even if a resist with relatively high resistance is used, minimum resist thickness for practical pattern fabrication is limited to approximately 50 nm. When the resist thickness is further reduced to get higher resolution, there occurs a problem that defects are generated on the processed substrate due to insufficient etching resistance of resist films, as shown in FIG. 3C.
Particularly, the acrylic main chain scission type resists have normally low etching resistance, because the main chain scission is also caused by etching reactive species; thus, it has been difficult to use this type of resist as a direct etching mask in a process including dry etching.
In addition, FIG. 7 shows a scanning electron microscope (SEM) photograph of a 0.05 .mu.m line-and-space pattern of thin resist. As clearly shown from the photograph, there has been a problem in that underexposed areas just near irradiated areas are partially dissolved during the development and a fine pattern having a vertical sectional form cannot be obtained.
Furthermore, when relatively thick resist of typically 0.2-0.5 .mu.m for the pattern fabrication of relatively large regime such as 50-150 nm is used to ensure sufficient etching resistance, the aspect ratio (i.e., a ratio of the pattern height to the pattern width) of the resist pattern must be increased to 4, or more. The inventors of the present invention found a problem that such high aspect patterns tend to collapse in a drying process of rinse solvent used for the development, because of the surface tension of the rinse solvent. As the collapse of high aspect ratio patterns is related to the mechanical strength of the resist film, in the case of resist thickness of about 50 nm, which is almost the minimum thickness for a practical nanometer fabrication process, very fine patterns around 10 nm wide tend to collapse since the aspect ratio is more than 4. Thus, the low mechanical strength of the resist film is a serious issue in nanometer pattern fabrication.
The inventors also paid attention to a time-dependent sensitivity problem in chemical amplification resist using an acid generated by irradiation of high energy beams as a catalyst as mentioned above. A polymer overcoat on the resist to stabilize sensitivity is inevitable in this case and this causes more complicated processes. Furthermore, in the case of thinner resist film which suffers more seriously from this problem, because deactivators such as water and ammonia can easily penetrate through the film, the polymer overcoat is not sufficient to completely stabilize the resist sensitivity, thus it is very difficult to perform highly precise pattern fabrication.